Structure, non-stoichiometry and thermodynamic properties ofBi1.85Sr2Co1.85O7.7−δ ceramics

O. Jankovsky a,∗, D. Sedmidubsky a, K. Rubesova a, Z. Sofer a, J. Leitner b, K. Ruzicka c,P. Svoboda d,

aInstitute of Chemical Technology Prague, Department of Inorganic Chemisty, Technicka 5, 166 28 Prague, Czech RepublicbInstitute of Chemical Technology Prague, Department of of Solid State Engineering, Technicka 5, 166 28 Prague, Czech

RepubliccInstitute of Chemical Technology Prague, Department of of Physical Chemistry, Technicka 5, 166 28 Prague, Czech Republic

dCharles University, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic

Abstract

The structure and oxygen non-stoichiometry of misfit layered cobaltite Bi1.85Sr2Co1.85O7.7−δ was determined byRietveld analysis and by thermogravimetric measurements. The heat capacity and enthalpy increments of pseudo-ternary oxide Bi1.85Sr2Co1.85O7.7−δ was measured by the relaxation time method (PPMS) from 2 K to 256 K, bydifferential scanning calorimetry (DSC) from 258 K to 355 K and by the drop calorimetry from 573 K to 1153 K. Aboveroom temperature the temperature dependence of the molar heat capacity in the form Cpm = (305.8 + 0.07325·T -4702536·T−2) J K−1mol−1 was derived by the least-squares method from the experimental data. The heat capacitywas analyzed in terms of a combined Debye - Einstein model. The molar entropy S◦

m(298.15) = 317.7 J mol−1 K−1

was evaluated from the low temperature heat capacity measurements.

Key words: misfit cobaltites, thermoelectric materials, heat capacity, Debye-Einstein model

1. Introduction

Due to the significant population growth in thelast decades worldwide energy consumption has dra-matically increased and, moreover, the major re-sources of fossil fuels are almost exhausted. The nu-clear power plants can provide enough cheap energy,but there are many risks connected with nuclear en-ergy. Other possibility how to produce electric en-ergy is the high-temperature thermoelectric conver-sion. Because the thermoelectric conversion is basedon the direct conversion of heat into electricity, thismethod is safe and simple. Thermoelectric batter-ies for high-temperature applications are composed

∗ Corresponding author, tel: +420 220 444 416Email address: JankovsO@vscht.cz (O. Jankovsky ).

of oxide materials due to their ability to generatethermoelectric power at high temperatures withoutthe risk to be decomposed and/or degraded by cor-rosion.Layered cobalt oxides with misfit structures are

the most promising candidates for thermoelectricconversion. Their structures are described by twosubsystems alternating along the c axis. A [CoO2]layer (CdI2 type), formed by CoO6 edge sharing oc-tahedra (rhombohedrally distorted) which is mainlyresponsible for the thermoelectric features and arock-salt block considered as an electron reservoir.These two blocks, which are alternatively stackedalong the c axis of the monoclinic lattice, share thesame a and c cell parameters but have different b1and b2 parameters incommensurate to each other.This family of compounds is called “misfit“ due to

Preprint submitted to Elsevier 10 February 2014

Fig. 1. Structure of the misfit layered systemBi2Sr2Co1.82O7.64

this incommensurability between the two subsys-tems. The incommensurate ratio of these subsys-tems can be calculated as q = b1/b2 [1].High coefficient of thermoelectric conversion was

achieved in [Ca2CoO3][CoO2]1.62 and [Bi2Sr2O4][CoO2]1.82 which can be expressed as Ca3Co3.93O9.36

and Bi2Sr2Co1.82O7.64, respectively. The first sys-tem, [Ca2CoO3][CoO2]1.62, has been well describedin the past. The phase equilibria of Ca-Co-Owere in-vestigated by Sedmidubsky et al. [2], the thermody-namic properties of Ca3Co3.93+xO9+δ were studiedby Jankovsky et al.[3] and the magneto-transportproperties were described by Hejtmanek et al.[4].Thermoelectric properties of Ca3Co3.93+xO9+δ wereimproved by using different routes of fabrication[5, 6, 7, 8] and by homovalent or heterovalent substi-tution [9]. The second phase, Bi1.83Sr2Co1.82O7+δ

(see Fig. 1), was discovered by Tarascon et al.[10] and its structure has been refined from X-raydiffraction data acquired on a single crystal [11].Thermoelectric properties of this material werestudied by A. Sotelo et al. [12, 13], while the studyof phase relations in the Bi-Sr-Co-O system has notyet been published. In the Bi-Sr-Co-O system, anumber of pseudo-binary and pseudo-ternary oxides[14, 15, 16, 17] has been identified. The pseudobi-nary system Bi-Sr-O was assessed by Hallstedt andGauckler [18] and we recently reported on the ther-modynamic behavior of Co-stabilized γ-Bi2O3 andthe liquid phase in the Bi-Co-O system [19, 20].This work represents part of a complex study

of phase equilibria in the Bi-Sr-Co-O system. Wefocused on the heat capacity, thermal propertiesmeasurement and the oxygen non-stoichiometryof [Bi2−xSr2O4][CoO2]1.85. This knowledge will beuseful not only for further calculations of the Bi-Sr-Co-O system, but also for the improvement of ther-moelectric properties by tailoring the microstruc-ture and intrinsic transport properties which isessential for further application of thermoelectric

batteries for thermoelectric energy recovery.

2. Experimental

Two samples of BiSrCoO misfit layer cobaltitewere prepared by conventional ceramic route and bychemical method (sol-gel route). The nominal cationratio in which the starting materials were mixed wasset to Bi/Sr/Co = 2/2/1.85 in both syhtesis routes.We assume that a higher reactivity of the green pow-der prepared by sol-gel route can facilitate reachingthe equilibrium phase composition of the resultingmaterial, though adjusting the proper compositionmight be more complicated in this case.The first sample (BSC-SS) was prepared by a solid

state reaction from pure powders of Bi2O3 (Aldrich,ACS), SrCO3 (Aldrich, ACS) and Co2O3 (Riedel-deHaen, ACS). The powders were mixed in an agatemortar, calcined twice in a platinum crucible at 973K and 1073 K in air for 48 hours each. After themanual homogenization the powders were pressedunder pressure of 0.5 GPa. The samples were sin-tered in air atmosphere (pO2/p

◦ = 0.21, where p◦ =101.3 kPa) at 1143 K for 96 hours.The second sample (BSC-SG) was prepared by

a sol-gel method based on the polyesterification ofethylenediaminetetraacetic acid (EDTA) and tri-ethanolamine (TEA) as described by Rubesova et al.[21]. The molar ratio of chelating agent 1:1:1 (met-als:EDTA:TEA) was chosen. Firstly Bi(CH3COO)3was dissolved in diluted acetic acid; then EDTAwas added and pH was increased using ammoniasolution until EDTA was fully dissolved; next, TEAwas added to this solution. The second solution wasprepared dissolving Sr(CH3COO)2 ·1/2 H2O andCo(CH3COO)2 · 4H2O in water; EDTA was addedand deprotonated using ammonia solution; TEAwas inserted at the end, as well. Both preparedsolutions were mixed together and heated at 353K. After water evaporation, the temperature wasincreased to 373 K to promote the esterificationprocess. After a while, the temperature started toincrease spontaneously up to 403 K and the solu-tion turned into gel. The gel was decomposed at523 K in air, then was heat treated in air at 973 Kand 1073 K for 2 hours. The prepared powder waspressed into pellets under pressure of 0.5 GPa, thenwere finally sintered at 1143 K for 96 hours in air.Both samples were analyzed by X-ray powder

diffraction (XRD). Data were collected at roomtemperature with Bruker AXS D8 Θ-Θ powder

2

diffractometer with parafocusing Bragg-Brentanogeometry using CoKα radiation. Data evaluationwas performed by means of the HighScore Plus soft-ware and by the Rietveld analysis using JANA2006software [22], suitable for misfit layered structures.Samples were analyzed by Scanning Electron Mi-croscopy (SEM) using TESCAN Lyra instrument.The elemental composition and mapping was per-formed with energy dispersive spectroscopy (EDS)analyzer X-MaxN with 20 mm2 SDD detector fromOxford instruments and AZtecEnergy softwarepackage.The sample BSC-SG was measured by simultane-

ous DTA a TG Setaram Setsys Evolution to 1373 Kto determine the temperature of melting and oxy-gen stoichiometry as a function of temperature. Theheating rate was set to 10 Kmin−1 and the measure-ment was carried out in dynamic air atmosphere.Moreover the oxygen non-stoichiometry was deter-mined from the mass loss upon reduction of mixedoxide at 743 K for 60 min in dynamic hydrogen at-mosphere (5 cm3 min−1) according to the followingequation:

Bi1.85Sr2Co1.85O7+δ(s) + (5+δ)H2(g) →

1.85Bi(s) + 2SrO(s) + 1.85Co(s) + (5+δ)H2O(g)(1)

The measurements of heat capacity were per-formed similarly to work [23] which was publishedby Leitner et al. The PPMS equipment 14 T-type(Quantum Design, USA) was used for the heatcapacity measurements in the low-temperature re-gion. The measurements were performed by therelaxation method with fully automatic procedureunder high vacuum (pressure ∼ 10−2 Pa) to avoidheat loss through the exchange gas. The sample wascompressed powder plates of approx. 5.3 mg. Thesample was mounted to a calorimeter platform withcryogenic grease Apiezon N (supplied by QuantumDesign). The procedure was as follows: first, a blanksample holder with the Apiezon only was measuredin the temperature range approx. 2 − 256 K to ob-tain background data. Then the sample plate wasattached to the calorimeter platform and the mea-surement was repeated in the same temperaturerange with the same temperature steps. The sampleheat capacity was then obtained as a difference be-tween the two data sets. The manufacturer claimsthe precision of this measurement better than 2%;the control measurement of very pure copper sampleconfirmed this precision in the temperature range50 − 250 K. However, the precision of the mea-

surement strongly depends on the thermal couplingbetween the sample and the calorimeter platform.A Micro DSC III calorimeter (Setaram, France)

was used for the heat capacity determination in thetemperature range of 258 − 355 K. First, the sam-ples were preheated in a continuous mode from roomtemperature up to 355 K (heating rate 0.5 Kmin−1).Then the heat capacity was measured in the incre-mental temperature scanning mode consisting of anumber of 5 K steps (heating rate 0.3 K min−1) fol-lowed by the isothermal delays of 2600 s. Two sub-sequent step-by-step heating were recorded for eachsample. Synthetic sapphire, NIST Standard refer-ence material No. 720, was used as the reference.The uncertainty of heat capacity measurements isestimated to be better than ± 1%.Enthalpy increments determination was car-

ried out by drop method using high temperaturecalorimeter, Multi HTC 96 (Setaram, France). Allmeasurements were performed in air by the al-ternating dropping of a reference material (smallpieces of synthetic sapphire, NIST Standard ref-erence material No. 720) and of a sample (smallpieces of pallets) being initially held at room tem-perature, through a lock into the working cell ofthe preheated calorimeter. Endothermic effects aredetected and the relevant peak area is proportionalto the heat content of the dropped specimen. Themeasurements were performed at temperatures 573K to 1153 K on samples of 200−250 mg. The delaysbetween two subsequent drops were 20− 25 min tostabilize heat flow. Estimated overall accuracy ofthe drop measurements is ± 3%.

3. Results and Discussion

The results of XRD analysis show small devi-ations of the phase composition of both samples.While the sample BSC-SG is single phase, the othersample BSC-SS contains small amounts ∼ 2%) ofCo3O4 and Bi0.75Sr0.25O1.36 (JCPDS 01-082-1059).This fact can be explained by different way of prepa-ration. Even though the sample BSC-SS was milledfor a long time, completely homogenous compositionwas not achieved and diffusion at relatively low tem-peratures (1143 K) is slow because of high porosity.The accurate pycnometric measurement cold not bedone in this case, but the density determined byweighting the sample pellets and measuring theirvolume reached only 60% of the theoretical densityin the case of BSC-SS sample while the BSC-SG was

3

Fig. 2. SEM micrographs of the BSC-SS sample (solid-statemethod, panel A) and BSC-SG (sol-gel method, panel B).The element distribution map from EDS is shown in panel C

more compact exhibiting the relative density of 71%(see comparison of sample microstructure in Fig. 2).The powder XRD data of a single phase sample

BSC-SG were subjected to Rietveld analysis as-suming the superspace group I2/a(0b0), fixing theatomic position to those reported by Leligny et al.[11] and releasing 24 paramaters (background, lat-tice parameters, peak shape, preferred grain orien-tation and Bi-occupancy). The resulting fit shownin Fig. 3 was obtained with a factor R = 0.167. Anextremely large preferred orientation of grains wasfound out documenting layered character of thestructure. Although the starting composition wasset to Bi/Sr/Co=2/2/1.85 the Bi site occupancy isslightly lower, 0.926(4), which is consistent with thesingle crystal refinement by Leligny et al. [11] whoidentified disordered regions within [BiO] layerswith modulated Bi-site occupancy. Since no otherphase was detected in the BSC-SG sample the orig-inal precursor composition must have been alreadyBi-deficient compared to weighted ratio. The Bi-defficiency was also confirmed by SEM-EDS yield-ing the overall stoichiometry (taken as an averagefrom 8 points) Bi1.89±0.04Sr2±0.03Co1.91±0.04O7.7−δ

(Sr content was fixed to 2). The structure ex-hibits the lattice parameters a = 4.944(1)A, bRS

1 =5.397(1)A, bCoO2

2 = 2.914(2)A, c = 29.889(1)A,α = γ = 90, β=93.50(1) and the modulation vectorq = b1/b2=1.852(4). The lowered Bi-site occupancyand the modulation vector determine the resultingcation stoichiometry Bi1.85Sr2Co1.85O7.7−δ.The oxygen stoichiometry of the slowly air-cooled

sample was determined from the mass loss upon

10 20 30 40 50 60 70 80 90

Inte

nsity

(a.u

.)

2

Fig. 3. Rietveld refinement of BSC-SG obtained by JANAprogram

the reduction in hydrogen atmosphere accord-ing to Eq. 1. The obtained value of δ = 0.071 inBi1.85Sr2Co1.85O7.7−δ is very close the ideal value(δ = 0) corresponding to a full occupancy of alloxygen sites. Hence we will consider the stoichiom-etry Bi1.85Sr2Co1.85O7.7 in the following evaluationof the measured heat capacity and relative en-thalpies data. This ideal oxygen stoichiometry andthe bismuth deficiency imply the cobalt valencyCo3.16+. Since the prepared material exhibits anearly temperature independent Seebeck coefficientα ∼ 145 µVK−1, we can compare this experimentalvalue with that obtained using Heikes formula sub-stituting Co4+/Co3+ = 0.16/0.84. The found valueα = (k/e)ln(0.16/0.84) = 143 µV K−1 corroboratesour results of bismuth and oxygen stoichiometryanalysis.The melting temperature was determined using

differential thermal analysis at 1177 K, hence theDROP method was used up to 1133 K. Thermo-gravimetric analysis showed that the sample lost1.25% of its mass on heating from 300 K to 1300 K.It can be assumed, that all the weight loss is causedby oxygen release (see Fig. 4 ). This amount cor-responds to the Bi1.85Sr2Co1.85O7.61 stoichiometry(δ = 0.09, cobalt valency Co3.06+) at 1177 K be-fore the melting and δ = 0.8 after the melting. Sincethe melting is likely incongruent the latter value ofδ corresponds to average cobalt valency Co2.3+ inthe mixture of oxide melt (comprising Co2+) anda second product of the peritectic melting with Covalency above 2+. The tubular structure cobaltiteBi4Sr12Co8O28 or non-modulated Bi221 phase withapproximate stoichiometry BiSr3CoO6 are the pos-sible candidates.The TGA data can be analyzed in terms of a sim-

ple oxygen non-stoichiometry model based on oxy-gen release from two [Bi0.925O] layers of fully satu-

4

600 700 800 900 1000 1100 1200 1300

Hea

t Flo

w (a

.u.)

Temperature (K)

1.0

0.8

0.6

0.4

0.2

0.0

1177 K

0.0008 0.0010 0.0012 0.0014 0.0016 0.0018-50

-45

-40

-35

-30

Rln

K (J

mol

-1 K

-1)

T-1 (K-1)

Fig. 4. DTA and TG analysis of BSC-SG sample measuredin air. The RlnK vs. T−1 dependence (K is defined in Eq. 2)

is shown in the inset for selected points of the TGA curve

rated composition Bi1.85Sr2Co1.85O7.7. The equilib-rium constant for such a reaction is defined as

K = e(∆HORT −∆SO

R ) =δp

1/2O2

2− δ(2)

where ∆HO and ∆SO are, respectively, the rel-ative partial molar enthalpy and entropy of oxy-gen. The oxygen non-stoichiometry coefficient δ inBi1.85Sr2Co1.85O7.7−δ then reads

δ =2K

K + p1/2O2

(3)

and its temperature dependence for a given value ofpO2

= 0.21 can be described by means of two param-eters, ∆HO = −15.21± 0.11 kJ·mol−1 and ∆SO =19.70 ± 0.12 J·mol−1·K−1. The linear dependenceof RlnK as a function of reciprocal temperature isshown in the inset of Fig. 4, where ∆HO and ∆SO

can be evaluated from the slope and ordinate inter-cept, respectively.Themeasured data used for heat capacity analysis

involve 113 Cpm values from relaxation time methodcovering the interval from 2 to 256 K, 40 points fromDSC from 258 to 355 K and 14 values of the enthalpyincrements from the drop measurements from 573 to1153 K. All Cpm data are plotted in Fig. 5 and 6 andthe enthalpy increment data are shown in Fig. 7.Although the material reveals an activated hoping

of charge carriers above ambient temperature, thecharge transport mechanism changes to metallic-like conductivity in the low temperature region. Theheat capacity thus involves both the electron and

0 50 100 150 200 250 300 3500

50

100

150

200

250

300

350

Cpm

(J K

-1 m

ol-1)

T (K)

RT Exp DSC Exp #1 DSC Exp #2 LT-fit

0 20 40 60 80 1000

100

200

300

400

Bi1.85Sr2Co1.85O7.7

T2 (K2)

Cpm

/T (m

J K-2

mol

-1)

Fig. 5. Low-temperature heat capacity of BSC-SG

phonon contribution. The electronic part of heat ca-pacity is considered in the form of Sommerfeld term

Cel = γT (4)

where the coefficient γ was fitted together withthe other parameters representing the lattice con-tribution to heat capacity. The resulting value ofγ = 28.1 ± 1.3 mJ mol−1 K−2 per formula unitor γ = 15.2 mJ mol−1 K−2 per Co is higher incomparison with γ = 5.4 mJ mol−1 K−2 per Co in[CoO2] block obtained by analogous procedure forCa3Co3.93O9.3 cobaltite with higher doping level(30% of Co4+) [3].The analysis of the lattice heat capacity was

performed within the harmonic approximation sim-ilarly to [3]. It is based on an additive combinationof Debye and Einstein modes. The phonon spec-trum of a polyatomic compound contains threeacoustic branches and 3n − 3 optical ones, wheren is the number of atoms per primitive unit cell.In our case, considering the structure formula[Bi1.85Sr2O3][CoO2]1.85, this represents 37.2 opticalbranches, which can be grouped into several opticalmodes described using characteristic Einstein tem-peratures ΘEi. Within this model the lattice heatcapacity is expressed as

CV =R

(3

xD

)3xD∫0

x4 · ex

[ex − 1]2dx

+R3∑

i=1

wEi · xEi · exEi

[exEi − 1]2, (5)

where xD = ΘD/T , xEi = ΘEi/T , and wEi =8.55, 12, 16.65 are the optical mode degeneracies.The three acoustic branches are taken as one triply

5

degenerate branch, which was attributed to Biatoms in the rock-salt block. Similarly, the rest of Biatoms and Sr atoms are supposed to contribute tothe first Einstein mode with the lowest frequency,the oxygen atoms in the rock-salt block to the sec-ond Einstein mode and the whole [CoO2] block tothe third Einstein mode with the highest frequency.Although this choice is rather arbitrary, it is partlysubstantiated by the fact that [CoO2] blocks aremore rigid and their atoms are thus supposed topredominantly contribute to high frequency phononmodes while the heavy and highly polarizable Biatoms participate in acoustic and low frequencyEinstein modes.The model parameters given in Table 1 (along

with the electronic specific heat coefficient γ) andthe resulting fitting curve shown in Fig. 5 arebased on a non-linear least-square fit applied onthe measured heat capacity data (from both therelaxation method and DSC). The absolute entropyof Bi1.85Sr2Co1.85O7.7 at the reference temperatureT = 298.15 Kwas evaluated by integrating the fittedCpm/T curve from T = 0 K to the reference tem-perature as S◦(298.15) = 317.2 ± 1.1 J mol−1 K−1.Similarly the integration of the fitted Cpm(T ) func-tion yields the relative enthalpy H◦(298.15) −H◦(0) = 48.13 ± 0.05 kJ mol−1.

Table 1

Parameters of Debye-Einstein model for lattice heat capacity(ΘD and ΘEi) evaluated by non-linear least-square fit usingthe selected degeneracies wi.

D E1 E2 E3

Θi 85.9 ± 1.3 158.8 ± 1.1 367.3 ± 1.4 759.3 ± 3.1

wi 3 8.55 12 16.65

A close inspection of heat capacity curve at lowtemperatures reveals a considerable deviation fromthe fitted curve (LT fit) below 10 K (see inset inFig. 5 in Cpm/T vs. T 2 representation). This dra-matic enhancement of Cpm can be likely ascribedto magnetic ordering or spin density wave forma-tion in [CoO2] blocks, however, detailed magnetiza-tion measurements or heat capacity measurementin magnetic field are necessary to confirm this as-sumption. Unfortunately, the exact integration ofthis excess term is not possible since we only seethe high temperature tail above 2 K, but a lin-ear interpolation to zero yields the entropy value∆S = 0.5 J mol−1 K−1 which is apparently underes-timated. A rough estimate of spin entropy assumingCo in low spin state (Co3+ spin S = 0 and Co4+ spin

0 200 400 600 800 1000 12000

50

100

150

200

250

300

350

400

Bi1.85Sr2Co1.85O7.7

3nR

Cpm

(J.m

ol-1.K

-1)

T (K)

RT Exp DSC Exp #1 DSC Exp #2 LT-fit HT-fit NKR Est

Fig. 6. Heat capacity of BSC-SG

500 600 700 800 900 1000 1100 120040

80

120

160

200

240

280

320Bi1.85Sr2Co1.85O7.7

THm (k

J m

ol-1)

T (K)

Drop Exp HT-fit

Fig. 7. Enthalpy increments of BSC-SG

S = 1/2) gives ∆Sm = 0.16Rln2 = 0.9 J mol−1 K−1

per Co site or 1.7 J mol−1 K−1 per formula unit.Let us note that below 1 K we should expect an-other Schottky-like peak on heat capacity curve re-lated to nuclear spin of Co, I = 7/2, giving riseto an appreciable entropy change ∆Sn = Rln8 =17.3 J mol−1 K−1 per Co atom. Nevertheless, theoverall entropy based merely on our experimentaldata, S◦(298.15) = 317.7 ± 1.1 J mol−1 K−1, isevaluated as a sum of electron, phonon and a de-tectable part of magnetic contribution.For the assessment of temperature dependence

of Cpm above room temperature, the heat capac-ity data from two independent DSC runs (fromthe temperature range 258 - 355 K) and the en-thalpy increment data obtained by drop calorime-try (temperature range 573 - 1135 K) were treatedsimultaneously by the linear least-squares method(HT-fit). The detailed description of the regres-sion procedure is given elsewhere [16]. The three-

6

parameter temperature dependence of heat capacityof Bi1.85Sr2Co1.85O7.7−δ in the temperature regionfrom 298 K to 1135 K can be expressed as

Cpm = [(305.8± 8.1) + (0.07325± 0.01657)T

−(4.70254± 0.30200) · 106T−2]Jmol−1K−1(6)

The comparison of the HT- and LT-fit curves(Fig. 6) clearly shows a divergence in high tem-perature limit. Since the harmonic approximationis believed to be valid below room temperature,the difference can be interpreted in terms of an-harmonicity effects. The anharmonic effect givesrise to thermal expansion of a material and thecorresponding Cp − CV term. Within our Debye-Einstein model a correction for anharmonicity canbe implemented using the anharmonic coefficientsαi by multiplying the respective Debye and Ein-stein terms by a factor (1 − αi)

−1 (see e.g. [16]).Unfortunately, due to linear temperature depen-dence of both the electronic specific heat and theCp − CV term the simultaneous fitting of γ andαi parameters is impossible due to high degree ofcorrelation. However, fixing the refined parametersfrom LT-fit and evaluating a single anharmonicitycoefficient αD = (4.14 ± 0.09)·10−4 K−1 using hightemperature Cpm data yields a reasonable fit. Theagreement can be even improved (comparable toHT-fit, Eq. 6) by releasing the second parameter,αE1, but in this case both parameters become againhighly correlated with unphysical value of αD < 0.As pointed out in our previous study of heat ca-

pacity of a related cobaltite Bi2Sr2CoO6.5−δ [16]the variable oxygen stoichiometry can substantiallyaffect the heat capacity measured at constant oxy-gen activity. Indeed the oxygen release from thecrystal structure into gaseous atmosphere drivenby equilibration of oxygen chemical potentials re-quires an additional heat to be supplied resultingin an excess (saturation) heat capacity, ∆satCpm =HO(∂δ/∂T ). However in the present case of misfitcobaltite, the temperature variation of oxygen sto-ichiometry δ (Eq. 3) is rather small resulting in arelatively small ∆satCpm which exhibits a flattenedmaximum of 1 J mol−1 K−1 at ∼ 600 K. Consider-ing the experimental error bar of high temperatureheat capacity determination this effect can be ne-glected in the present analysis of thermal behaviorof Bi1.85Sr2Co1.85O7.7−δ.

4. Conclusions

In this study, we have successfully synthetizeda single-phase misfit-layered cobaltite whose stoi-chiometry was determined as Bi1.85Sr2Co1.85O7.7−δ

with a variable oxygen content δ ∼ 0-0.09 in airatmosphere. The thermochemical properties of thismaterial were measured by three calorimetric tech-niques applied in different temperature ranges, andthe temperature dependence of heat capacity wasevaluated considering the fixed oxygen stoichiome-try, δ ∼ 0. This assumption was based on the as-sessment of the partial molar enthalpy of oxygen,which was found to have a negligible effect on the re-sulting heat capacity. Moreover, the melting behav-ior of Bi1.85Sr2Co1.85O7.7−δ was examined by DTAand the peritectic melting temperature at 1177 Kwas accompanied by a sudden release of oxygen.Such a high melting temperature and a relativelyrigid structure with a minute variation of oxygenstoichiometry makes the material suitable for fur-ther applications in thermoelectric devices for high-temperature thermoelectric energy recovery.The obtained heat capacity was analyzed in terms

of a combined Debye-Einstein model at low temper-atures and by a polynomial form at elevated tem-peratures. This knowledge could by useful for theimprovement of thermolectric conversion efficiency;however, it is essential for the thermodynamic studyof the quaternary Bi-Sr-Co-O system, thus our fu-ture effort will be focused on phase equilibria mod-eling and phase diagram construction in the Bi-Sr-Co-O system in order to improve the thermoelectricproperties of p-type materials in thermoelectric bat-teries.

Acknowledgement

This work was supported by Czech Science Founda-tion,Grant No. 13-17538S, and by Ministry of Edu-cation of the CzechRepublic, Grant No. 20/2013 forspecific university research. Experiments were per-formed in MLTL (http://mltl.eu/), which is sup-ported within the program of Czech Research In-frastructures (project no. LM2011025). The work ofP. S. was supported by Czech Science Foundation ofthe Czech Republic, Grant No.P108-10-1006.

7

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